Bonded Exciplexes: A Fruitful Collaboration between Experiment and Theory
Professor Joseph Dinocenzo
Department of Chemistry, University of Rochester, Rochester, NY 14627-0216, USA
Lundi, 11 octobre 2010,
11h00 Salle RAPHAEL 2&3
site d Ivry-sur-Seine
Charge-transfer quenching of the singlet excited states of cyanoaromatic
electron acceptors by pyridine is characterized by a driving force dependence
that resembles those of conventional electron-transfer reactions, except
that a plot of the log of the quenching rate constants vs. the free energy
of electron transfer is displaced toward the endothermic region by 0.5-0.8 eV.
The reactions therefore display rapid quenching even when conventional
electron transfer is highly endothermic. No long-lived ion radicals or
exciplex intermediates can be detected for the quenching reactions on the
picosecond to microsecond time scale. The reactions are proposed to instead
proceed through formation of a previously unrecognized, short-lived
charge-transfer intermediate we call a "bonded exciplex". The bonded
exciplex can be formally thought of as resulting from bond formation between
the unpaired electrons of the ion radicals A-- and D+-. The bonding
interaction significantly lowers the energy of the charge-transfer state.
As a result of this interaction, the energy decreases with decreasing
separation distance and, near van der Waals contact, the charge transfer
configuration (A--D+) mixes with the repulsive excited state of the acceptor
(A* + D) allowing efficient charge transfer reaction even when formation of
A-- and D+- is thermodynamically forbidden. Experimental and computational
evidence for the formation of bonded exciplexes will be presented.